This is a proposal to continue and conclude a randomized, double-blind clinical trial of the efficacy of dietary supplements in preventing neoplastic polyps (adenomas) of the colorectum in persons at high risk for this condition, It is a collaborative investigation conducted at six clinical centers: Cleveland Clinic, Dartmouth Hitchcock Medical Center (also the coordinating center), Lahey Clinic Medical Center, University of California at Los Angeles, University of Iowan and University of Minnesota. The study involves over 850 patients who were enrolled shortly after diagnosis and removal of a large bowel adenoma. They were assigned at random to four treatment groups in a 2x2 factorial design with the test treatments consisting of (1) 30 mg of beta-carotene daily, (2) the combination of 1 gram ascorbic acid and 400 mg alpha tocopherol daily, (3) both treatments (i.e. beta_carotene and combined ascorbic acid and alpha tocopherol) or (4) placebo only, Participants come to the clinical centers or are contacted by telephone or mail at three-month intervals for capsule distribution and assessment of compliance and toxicity. Blood is drawn and analyzed for beta-carotene retinol and alpha tocopherol at study entry and at annual intervals thereafter. Blood chemistry including SGOT, BUN, creatinine, glucose and total cholesterol is determined at entry and after one year. Selenium, zinc, and ferritin are measured on the blood specimens obtained at entry only. At 12 months and 48 months after entry participants receive a complete colonoscopy with removal and micoscopic examination of all poly. The primary statistical analysis will assess treatment effects by comparing the proportions of patients who develop a new adenoma in the three years between the 12-month and 48-month follow-up examinations. This request is for the final four years of the planned nine-year study.